How to optimise hypersonic flight in FAR

[AeroGav]

As I understand it,  there are various kinds of drag an airplane can encounter.   It is not possible to optimise for all in a single design,  so it is necessary to set priorities based on the role of your aircraft -

1.   Parasite drag.  Same as your car experiences.   Dominant in high dynamic pressure reigimes.  Minimise wing span to reduce.

2.   Lift induced drag.  Worst at high angles of attack, ie. low dynamic pressure reigimes. Having a high aspect ratio wing (long span but comparatively thin, like a glider) reduces lift induced drag, as does having sufficient wing area to avoid high angles of attack.

3.   Wave drag.   Dominant in the transonic flight regieme.  Optimise your area ruling to minimise.

4.  Shockwave drag ?  Significant at supersonic speeds.  Keep the wing tips and basically the entire aircraft behind the "bow wave" created by the aircraft's nose, to minimise this.

 

Now there are other aerodynamic qualities an aircraft might design for, but I'm not going to include roll rate or supermanuverability since this is all about space planes not fighters.   Of the above, 1) is not so important to me either because of the way I fly - i don't try to go fast until i'm really high up, at which point the air is so thin,  i need a large wing area to avoid  2)  more than i need small wing span to avoid 1).

The trouble with FAR, at least to a noob like me, is that Transonic Wave drag is very much thrown in your face,  and I find myself becoming a slave to it.  Optimise the heck out of the problem you can see,  and forget about Lift Induced and Shock Drag because whilst they are modelled, they are buried deep in the UI.

To be more specific, I always end up optimising so that the Mach 1 Wave Drag area is less than 20% of the Max Cross Sectional Area of the aircraft, sometimes much less.    However, so long as it's 30% or less you're likely to find penetrating the sound barrier easier than with stock aerodynamics in 1.05, and certainly much easier than with the brick wall you faced in 1.04.

I'm unsatisfied with the aircraft I've created so far because I'd like to transfer some of their excessive performance in the middle part of the speed range, transonic,  into better performance at the bottom and top end.

Why am I bothered about low and slow?   Because I like to set up IRSU infrastructure and have my space planes refuel and go beyond Kerbin to  other bodies, eg. Duna, Laythe.   There they will need to make off-airport landings, and reducing the stall speed is vital to accomplish this in one piece.

Why am I bothered about hypersonic flight through the Mesosphere?  

For the way I operate, this is the most critical regime of all.   A lot of people fly like an airplane up to 20km/Mach 4, then transition to thrust-borne flight and zoom up to orbit at TWR > 1 in a cloud of burnt LF/O.    I tend not to use any oxidizer at all however and after getting as high as possible in airbreathing mode, light up the NERV engines simply keep flying ever higher on their meagre thrust.   So long as I can keep a good lift/drag ratio above 30km and mach 4,  thrust will exceed drag and i'll slowly gain kinetic/potential energy.   As we get ever higher, lift will decrease, but since speed is increasing, the centrifugal force of hurtling around the planet will cancel out more and more gravity.

What's our design point?      Well, up to 30km/Mach 4 I still have RAPIERS , so this is the lower boundary.          And by 50km/Mach 7 I usually shut down and coast to an Apoapsis over 70km,   so we can say the mid point is  Mach 5.5 and 35/40KM.

How do you optimise for low drag here, which factors matter?  Is transonic/area ruling still the most important?    Does the aspect ratio of your wings still matter at all?  Presumably we still need plenty of wing area to keep our AoA down to reasonable levels in what is going to be VERY thin air?

Lastly, is it at all possible to have the above with also a reasonable landing speed, by sacrificing other flight areas  I don't find so important?   Certain real world strategies aren't available to us in game,  eg. Variable Geometry wings,  flap blowing etc. but we shouldn't complain too loudly as orbital velocities in KSP are only 1/3 of real life, greatly reducing the speed range we have to get our designs to perform under.

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This is an example of my last design under FAR with Procedural Wings.

My reasoning went thus -

This is the aircraft that resulted.  I had Interstellar mod installed,  so that thing you can see poking from the forward cargo bay is a pebble bed reactor that doesn't quite fit.    At the rear of the fuselage is a hybrid thermal turbojet/nuclear thermal rocket.   It operates as a nuclear turbojet in the lower atmosphere, then as a nuclear thermal rocket higher up.   Buried in the cargo bay is a mk1 cockpit whose canopy clips through the bay doors, and a mk1 passenger cabin.     Crew capacity - 3.

I'm not sure what most of these numbers mean, but i've been told that green is good.  At sea level this design needs AoA of 4.2 to maintain level flight @ 120 m/s, which is as good as anything i've made in FAR.   The numbers on this screen don't seem to be affected by me lowering the flaps, but they do seem to have an effect when actually flying.   With full fuel, I can stay airborne down to 70 m/s at about 10 degrees nose up.  After that the canard stalls and the nose drops.  I suppose there's not much point going to greater pitch anyway, it would just result in a tailstrike.    In practice we'd be landing with little or no fuel.  I've found that 3 Vernier engines translating upward can substantially lower airplane landing speeds on Duna.

EDIT - oh yes, the main point.  The best L/D ratio I seem to be able to get with this airplane is 3, in the >Mach 4, high altitude regime.  To me , that sounds very low, but can it realistically be made better?  

 

 

Edited January 9, 2016 by AeroGav